1. Field of the Invention
This invention relates generally to holographic optical elements and more particularly to a new method and apparatus in which a transmission holographic optical element is constructed of two reflection holographic optical elements placed together in close proximity.
2. Description of Related Technology
A hologram created by the interference of a plane reference wave and a divergent spherical wave will, when illuminated by a similar planar wave, create a divergent spherical wave (the virtual image) and, when illuminated by an oppositely directed planar wave will create a convergent spherical wave (the real image). Such a hologram behaves as a lens and can therefore be characterized as an optical focusing device. Since such a hologram is used to control the path of a light beam rather than to display an image it is referred to as a holographic optical element. The main advantages of holographic optical elements are that they are lightweight, compact and thin when compared to conventional lenses.
In general, the diffraction efficiency (.eta.) of an optical element is defined as the percentage of incident light that is diffracted into the image field: EQU .eta. (%)=(D/I) 100
where D is the intensity of diffracted, image forming light, and I is the intensity of incident light.
In a conventional transmission holographic optical element, the diffraction efficiency is a periodic function of index modulation, and can be expressed approximately by the periodic function: EQU .eta.=sin.sup.2 (.DELTA..phi./2),
where .DELTA..phi.=2.pi. t.DELTA.n/.lambda. cos .theta., PA1 t=is the gelatin thickness, PA1 .DELTA.n=the refractive index modulation, PA1 .lambda.=the wavelength, and, PA1 .theta.=the angle of incidence.
For a reflection hologram, diffraction efficiency .eta.=tanh.sup.2 (.DELTA..phi./2), which therefore increases with an increase in either the gelatin thickness or modulation. In principal, the saturation value is 1 (i.e., 100%). In reality, the gelatin absorbs some light and so the saturation value is somewhat less (&gt;95%). In the laboratory, refractive index modulation .DELTA.n is inferred from measurements of diffraction efficiency. For a transmission hologram, the two vectors representing the incident and diffracted light are nearly parallel and any change in their length (i.e. wavelength) has very little effect on the optimal grating vector, so diffraction efficiency remains high over a broad range of wavelengths. For a reflection hologram, the incident and diffracted light vectors are nearly antiparallel and any change in their length has a very strong effect on the optimal grating vector and thus diffraction efficiency is highly wavelength dependent.
A commercial application of such devices is as the combiner within a "HEADS UP DISPLAY", commonly used on certain types of military aircraft and vision enhancement devices.
A conventional transmission holographic optical element is undesireable for use as a combiner because its "see-through" characteristic is poor.
A "Heads Up Display" using holographic tuned reflective optical coatings is disclosed in U.S. Pat. No. 4,261,647, issued to Ellis. The reflections in the Ellis patent are strictly specular: angle of incidence equals angle of reflection. This necessitates a space of significant wedge between the elements so that the two encounters of the light beam with either element are not parallel; they must differ in angle by at least the angular bandwidth of the reflective coatings. A wedge space of air introduces two additional air-glass interfaces which can scatter light. A wedge space of glass makes for a very heavy assembly. Moreover, any wedged space makes for a complicated optical design as it introduces difficult to compensate off-axis aberrations. Because the two elements of our sandwich are holographic, angle of incidence need not equal angle of diffraction and the geometry therefore will not require a wedged space.